1. Field of the Invention
The present invention relates to a tracking control apparatus and, more particularly, to a tracking control apparatus for use with magnetic tape having a video track and a control track in a magnetic recording and reproduction apparatus, such as a video tape recorder.
2. Description of the Related Art
In general, household video-tape recorders (referred to as VTRs hereinafter) use rotating magnetic heads for recording and reproducing video signals. These VTRs are provided with a head servo circuit for controlling rotating speed and rotation phase of the heads and a capstan servo circuit for transporting magnetic tape at constant speed at a time of recording and in synchronization with a recorded control signal at a time of reproduction. The control signal is recorded on the tape in synchronization with video signals being. However, control and video heads are different in their mechanisms. Hence the phase relationship between the control signal and the video signal varies according to mounting positions of the control head and the video heads. This variation will cause tracking error which may appear on a TV picture-tube screen at the time of reproduction of video signals.
To keep VTRs compatible, the record phase of the control signal is set up as a standard and is thus adjusted at a stage of manufacture of the VTRs. In the capstan servo circuit, on the other hand, in case where a phase shift occurs in the recorded control signal, the phase of a reference signal for a phase comparator in the servo circuit is changed by a delay circuit, thereby compensating for the phase shift of the control signal. However, such compensation is performed by a user while watching VTR reproduced pictures on a TV picture-tube screen. That is, to obtain the most pleasant pictures, the user adjusts an externally accessible variable resistor installed to a VTR body, which is called a tracking volume control, while watching the reproduced pictures.
Recently there has been proposed a method of automatically compensating for the phase shift of a control signal described above by a tracking control apparatus using a microcomputer as shown in FIG. 1. A reference signal generator 12 generates a reference signal a for controlling a capstan motor and a head-drum motor described later. The reference signal a is applied to an input port 14.sub.1 of a microcomputer 14 and a monostable multivibrator 16. Control head 18 is adapted to reproduce a control signal from magnetic tape. After being amplified in an amplifier 20, the reproduced control signal is applied to a Schmitt circuit 22. The reproduced control signal c wave-shaped by Schmitt circuit 22 is applied to a phase comparator 24 together with the reference signal b which has been subjected to a predetermined time delay by monostable multivibrator 16. Phase comparator 24 is adapted to make a comparison in phase between the reference signal b and the reproduced control signal c and provide to a mixing amplifier 26 an output signal d which represents the phase difference between both the signals.
An FG coil 28 generates an FG signal proportional to the rotation frequency of capstan motor 30. The FG signal is fed to an amplifier 32 for amplification. The output signal of amplifier 32 is applied to a Schmitt circuit 32 for wave-shaping and then applied to a frequency discriminator 36 as an FG signal e. Frequency discriminator 36 measures the frequency of the FG signal e and applies a speed signal f to mixing amplifier 26 accordingly. Mixing amplifier 26 mixes the phase difference signal d and the speed signal f together to produce a motor control signal g which is fed to a motor drive amplifier 38. Motor drive amplifier 38 amplifies the motor control signal g to provide a signal for rotating capstan motor 30.
A video signal reproduced by a video head 40 is applied to a peak detector 44 via a preamplifier 42, which detects a peak level of the reproduced video signal for application to an A/D conversion input port 14.sub.2 of microcomputer 14.
Microcomputer 14 converts an output of peak detector 44 to a digital signal. A transistor 46 is connected to an output port 14.sub.3 of the microcomputer 14 via a resistor R.sub.1 as shown in FIG. 1.
Monostable multivibrator 16 comprises NAND gates 162, 164, a comparator 166, a transistor 168, a resistor R and a capacitor C forming together a delay circuit, a resistor R.sub.2 and a capacitor C.sub.1. NAND gate 162 has an input connected to reference signal generator 12 and the other input connected to the output of NAND gate 164. NAND gate 162 has an input connected to the output of NAND gate 162 and the other input connected to the output of comparator 166. NAND gate 164 provides reference signal b to phase comparator 24 and to transistor 168 via resistor R.sub.2. Comparator 166 has its noninverting input (+) connected to ground via capacitor C.sub.1 and its inverting input (-) to the junction of resistor R and capacitor C, which is also connected to transistor 46.
In operation, reference signal a generated by reference signal generator 12 at time t.sub.1 as shown in FIG. 2A is applied to microcomputer 14 and monostable multivibrator 16. In monostable multivibrator 16, reference signal a is delayed by the delay circuit formed of resistor R and capacitor C, the delay time depending on a time constant defined by resistance and capacitance values. In response to the trailing edge of reference signal a, transistor 168 in monostable multivibrator 16 is turned off and output port 143 of microcomputer 14 goes high, thereby causing transistor 46 to be turned on as shown in FIG. 2B. The inverting input (-) of comparator 166 thus goes low so that the output thereof becomes high. Therefore the NAND gate 164 goes low as shown in FIG. 2D.
Microcomputer 14 receives the output of peak detector 44 at input port 142 and causes output port 14.sub.3 to go high when a peak level is detected with the result that transistor 46 is turned off (time t.sub.2). The time interval between t.sub.1 and t.sub.2 is a delay time for the reproduced signal to attain the maximum level. As a result of transistor 46 being turned off, such a pulse as shown in FIG. 2C is applied to the inverting input (-) of comparator 166. The pulse rises in accordance with the time constant defined by resistor R and capacitor C in the monostable multivibrator 16, thereby causing the output of comparator 166 to go low. Consequently the output of NAND gate 164 goes high at time t.sub.3 as shown in FIG. 2D. Once the output of NAND gate 164 goes high, transistor 168 is turned on, so that the high level output of NAND gate 164 is held until reference signal a next goes low at time t.sub.4 as shown in FIG. 2D.
Thus reference signal b output from NAND gate 164 is kept in sync with the peak level of the reproduced video signal detected by microcomputer 14. Hence the rotation phase of capstan motor 30 is fixed at a time when the peak level is detected, thereby completing the tracking adjustment.
According to the tracking adjustment method described above, however, the operating point of monostable multivibrator 16 is delayed by microcomputer 14. Consequently there arises a problem that synchronizing error occurs because the amount of delay has to be internally generated by microcomputer 14.
Moreover, there is another problem that the connection of microcomputer 14 to the servo system increases the component count and manufacturing cost.